1. Field of the Invention
The present invention is concerned with new photoresists for use in the manufacture of microelectronic devices such as those used in microelectromechanical systems (MEMS).
2. Description of the Prior Art
It is common in silicon etching processes to utilize a thin (100- to 300-nm) silicon nitride or silicon dioxide coating on the silicon substrate as a mask for patterned etching or as a passivating layer to enclose active circuitry. In the prior art, etch protective coatings or masks for MEMS fabrication processes have been selected primarily by using a trial-and-error method because there are no general purpose protective coatings on the market. The etch selectivity of the etchants to various materials is often used as a guide for MEMS process engineers. With a much lower etch rate than silicon, films of silicon nitride have been used as a protective layer or hardmask during KOH or TMAH bulk silicon etching. Silicon dioxide has a higher etch rate than silicon nitride. Therefore, it is only used as a protective/mask layer for very short etches. Gold (Au), chromium (Cr), and boron (B) have also been reportedly used in some situations. Non-patterned, hard-baked photoresists have been used as masks, but they are readily etched in alkaline solutions. Polymethyl methacrylate was also evaluated as an etch mask for KOH. However, because of saponification of the ester group, the masking time of this polymer was found to decrease sharply from 165 minutes at 60° C. to 15 minutes at 90° C.
Regardless of the protective coating or mask selected, a photoresist layer to be patterned must be applied to the protective coating or mask so that the pattern can be transferred to the underlying substrate. However, this can only be carried out after the protective coating or mask has been applied, thus requiring time and expense to apply and later etch this protective layer or mask, which is very difficult to remove.